Corrosion has a significant impact on the performance and reliability of U.S. infrastructure, transportation, utilities, production and manufacturing. Gerhardus reports that the total direct cost of corrosion to the U.S. has been estimated to be $279 billion per year, which is 3.2 percent of the U.S. gross domestic product. Critical needs have been identified for new technologies or corrosion management techniques including advance monitoring and detection systems for corrosion management, life prediction and performance assessment.
Numerous engineering materials in common use are degraded through corrosion processes. For example, metals and alloys such as steel and aluminum often corrode when exposed to industrial or marine atmospheric environments. Such corrosion processes often cause damage to equipment and structures fabricated from these materials, leading to reduced operational life and reliability. To minimize the undesirable consequences of corrosion on product usability, reliability, and lifetime, it is often desirable to repair or maintain corroded materials before they reach a critical level of damage. For example, some portions of vehicles are more prone to corrosion than others; the bottom of vehicles often has more corrosion than the top. In this case, it is desirable to repair or repaint the areas along the bottom and underside before they degrade to the point that the vehicle function and/or reliability is adversely affected.
Low cost and easy to use corrosivity sensors are required to manage corrosion and reduce life cycle costs, improve performance, and minimize corrosion related failures. Knowledge of cumulative environmental severity, corrosion damage accumulation, and corrosion protection system breakdown would enable more effective prediction and management of asset life, performance and maintenance. The corrosion monitoring system would provide field performance data on new coatings and paint systems and would be a means for determining the effectiveness and return on investment of corrosion prevention and control programs. The requirements for a corrosion monitoring system to be successful are: low cost, small size and weight, wide deployment capability, easy installation, simple operation and data analysis and low maintenance.
Various measuring methods may be used to detect and monitor corrosion. One of the most basic methods requires placement, exposure, and retrieval of mass loss coupons. Another class of sensors requires direct connection to the sensing device including electrical, fiber optic and possibly acoustic devices that may measure either material loss, corrosion rate or environmental parameters that promote corrosion. An automated variant of mass loss coupon measurements is the electrical resistance probe. Electrochemical measurement techniques include electrochemical impedance spectroscopy, electrochemical noise, galvanic currents, redox potential, and linear polarization resistance methods. Similarly, chemical and environmental sensing devices may measure temperature, ion concentration, pH, dissolved oxygen, moisture, etc.
Electrical and fiber optic sensor techniques and apparatus require direct connection to the sensor, and therefore, require that the reader used to determine the state of the sensor be placed in direct physical contact with the sensing element. This requires the use of wires or fiber running from the corrosion sensor to a powered reader device. The need for a direct electrical or physical contact is a serious drawback, as it limits the placement of sensors to regions where wire can be routed or direct connection can be made. In addition, the process of making these connections and associated measurements is difficult and time consuming. To overcome these limitations systems have been developed that contain power, processors and wireless transceivers that can be located near or with the sensing element. This increases system complexity requires dedicated communication and processor hardware for each sensing element and sensor power. These actively powered devices may have limited service lives and require maintenance such as battery replacement.
A basic corrosivity measurement technique uses mass loss samples that are placed in the environment of interest. This technique is detailed in ASTM G1-03 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. This technique is simple and reliable, but requires that the samples be retrieved periodically for cleaning and mass loss measurements. Also, the accuracy of the measurement depends on the mass of the sample and the total amount of material removed by corrosion. Accurate measurements may require very long exposure times of months to years. The long sampling intervals, need for sample racks, and laboratory measurement of weight loss all restrict the use and limit the value of this measurement technique.
Electrochemical sensor measurements include electrochemical potential, linear polarization resistance, electrochemical impedance spectroscopy, electrochemical noise measurements, and galvanic current measurements. These techniques all measure the instantaneous conditions of electrodes to determine corrosion conditions or corrosion rates. These methods are highly dependent on the conductivity of the surrounding environment and will not measure corrosion in the absence of a conductive medium. A significant disadvantage of these techniques is the lack of a direct measure of cumulative corrosion damage. Indirect measurement of total corrosion can be obtained only through a continuous monitoring of the corrosion rate. This requires dedicated data collection and processing hardware and software.
Measurements of environmental conditions or chemistry such as temperature, pH, time of wetness, relative humidity, and ion concentrations have been used by Srinivasan U.S. Pat. No. 6,796,187 and Watters U.S. Pat. No. 7,034,660 to infer corrosivity of a given environment. These are indirect methods that depend on empirical equations to estimate corrosion and do not measure the cumulative corrosion damage or corrosion rate. As with electrochemical measurements, these are instantaneous condition measurements, and the only way to infer cumulative environmental corrosivity and predict total corrosion is to continuous collect and record data that can then be used in models that predict damage state based on environmental parameters.
The electrical resistance technique is another method for corrosion measurement. This measurement is based on the change of resistance of a sensing element as it corrodes or is otherwise damaged by the environment. The change, e.g., diminution of size of the object, increases the resistance of the metallic specimen and, therefore, directly relates to the loss of metal by corrosion and/or erosion. The data can be converted to unit loss of metal per time unit to provide corrosion rate per year or similar time period. The resistance measurement requires the direct connection of an ohmmeter or some equivalent measurement device to the sensor. The measurement range and sensitivity is dependent on the length and cross sectional area of the sensing element. In general, long thin patterns of metallic conductors are used to achieve the desired service life and desired damage sensitivity. Changes in resistance of the specimens due to diminution of mass are small, and range within milli and micro-ohms. As such, signals with sufficient noise levels are difficult to obtain and readily subject to extraneous influences. Should the sensing element be perforated and the conductive path interrupted the sensor will not function. These sensors have poor resolution during the initial stages of corrosion, and to obtain reasonable service life, the corrosion needs to be a uniform general attack, not localized.
Other measurement methods by Tiefnig U.S. Pat. No. 6,919,729 are described that use changes in inductance of a current carrying coil to measure corrosion damage. In these techniques, a coil contains or is in proximity of a mass of metal that is the sensing element. As the metal is removed by corrosion the inductance of the coil changes and this can be used as a measure of corrosion damage. The technique requires direct connection of the coil to a power source to make the measurement. The need for this direct connection is a disadvantage for use under coatings or in situations where leads are inconvenient such as on the exterior of a vehicle, or where leads and connections may also be damaged or corroded.
A variety of wireless devices powered externally are described in the literature. A number of these methods require that the sensor be interrogated to determine the resonant frequency or Q factor using an external device reader. These methods involve sensing elements within the circuit that when altered by the physical affect to be monitoring the resonant frequency of the circuit is modified. This method constrains the sensor design to structures that produce changes in the resonant frequency as a function of corrosion damage, must have provisions for determining the resonant frequency with a wireless reader, and have the sensing element electrically connected to an electrical circuit.
The wireless device of Varpula US 2007/0241762 for monitoring environmental conditions using an inductive element that is not electrically (galvanically) connected to the resonant circuit requires determination of the resonant frequency or quality (Q) factor for detecting environmental contaminants. It also does not provide for reference measurements, correlation to structural conditions, identification and data storage, or operation on metal substrates.
Woodard U.S. Pat. No. 7,086,593 discloses a resonant electrical circuit that senses physical changes in the environment by changes in the capacitive element of the electrical circuit. Changes in the capacitive element of the circuit alter the resonant frequency of the circuit. The sensing element is formed from metal conductors that have parallel plates or interdigitated printed pattern geometries. The capacitance change is dependent on environmental interaction with the dielectric medium between the two capacitor electrodes. This device does not directly sense corrosion and is therefore only capable of monitoring conditions that may cause corrosion. Also, without continuous monitoring, no historical measure of cumulative damage is possible.
Another device by Subramanian US 2006/0125493 discloses the use of a coil wirelessly coupled to an antenna that can be used to power a corrosion sensor. The resistance of the sensing element is a function of the corrosion damage and this property change is used to measure corrosion damage. As with traditional resistance sensors, the measurement range and sensitivity is dependent on the length and cross sectional area of the sensing element. In general, long thin patterns of metallic conductors are used to achieve the desired service life and desired damage sensitivity. Should the sensing element be perforated and the conductive path be interrupted the sensor will not function. These sensors have poor resolution during the initial stages of corrosion and to obtain reasonable service life the corrosion needs to be a uniform general attack, not localized.
The above methods for monitoring corrosion by wireless sensing devices have several disadvantages. These methods do not include a reference sensor in the sensor design. Without a reference sensor, it is difficult to quantify the amount of environmental damage seen by the sensor. Each measurement of the sensor requires controlling every variable that influences the response of the sensor to interrogation including distance between the sensor and interrogator, polarization of the sensor relative to the interrogator, and environmental factors not being sensed by the sensor. The methods based on resonant frequency response require many measurements to be taken over a wide range of frequencies. Taking and analyzing this data is computationally intense and time consuming. Finally, methods for treating the case when the monitored structure is a conductor is not described.
Sirkis U.S. Pat. No. 5,367,583 discloses the characterization of corrosion using optical sensing techniques. The corrosion sensor acts as a mirror in a Fabry-Perot cavity; corrosion-induced changes in reflectance are measured optically to determine the extent of corrosion. Udd U.S. Pat. No. 6,144,026 discloses an optical corrosion characterization technique. Corrosion sensor systems are formed by using one or more fiber gratings whose transverse strains vary with corrosion or chemical attack. By optical probing, it is therefore possible to determine the corrosion along the fiber. Optical techniques require that the sensor be physically connected to the interrogation hardware. These measurements provide only limited information on material loss and corrosion rate and are computationally intensive to convert the optical signal to corrosion damage. The need for direct connection precludes the use of this device under coatings or in harsh environments where the connectors can be fouled or damaged.
In view of the many drawbacks associated with corrosion sensing devices and techniques, there is a need for a corrosivity sensor and monitoring system where the output correlates to cumulative exposure and damage accumulation of a component, vehicle or structure, that is simple in design and operation, easy to read, does not require direct electrical contact to the sensing element, and can be used on metal structures.